1. Field of the Invention
This invention relates to techniques for improving the dynamic range of analog sensing arrays and, more particularly, to a digital interface circuit which avoids the noise produced by analog multiplexing circuitry used to sample individual detectors in a sensing array.
2. Description of the Prior Art
In employing wide dynamic range analog detector arrays (both linear and area arrays) for detecting patterns of energy impinging thereon, direct sensing of the detector outputs conventionally requires analog multiplex circuitry to sample the individual detectors in the array. This analog multiplex circuitry has limited dynamic range, and often introduces Johnson noise and switching noise. The dynamic range of this analog multiplex circuitry is usually significantly lower than the dynamic range of the detectors being sensed and, consequently, limits system dynamic range. Introduction of amplifiers ahead of the multiplex circuitry improves performance, but the full dynamic range of the detector-amplifier combination is still not available.
One-dimensional (linear) and two-dimensional (area) image detector arrays have been developed using photodiode, charge injection device (CID), and charge coupled device (CCD) technology. The signals produced by the detectors in these arrays are delivered to one or more output nodes using some type of analog multiplexer, usually comprised of either switches or charge transfer devices. The limit on system performance imposed by the multiplexer is a result of the capacitance load of the multiplexer circuitry, Johnson noise introduced by the sampling function, switching interference, or, in the case of charge transfer, the introduction of charge transfer losses and charge transfer noise.
The technical literature is replete with descriptions of image detector arrays employing amplifiers ahead of analog multiplex circuitry for sampling individual detectors of the arrays. For example, in G. J. Michon U.S. Pat. No. 4,807,038, issued Feb. 21, 1989 and assigned to the instant assignee, low noise, wide dynamic range amplifiers are integrated with the sensing array prior to the analog multiplex function. The Michon apparatus has been designed with particular care to include a low noise, wide dynamic range analog multiplexer following the amplifiers; however, in the resulting sensor system, which has a dynamic range in excess of 60 dB, the dynamic range (and readout rate) remains limited by noise introduced by the analog multiplexer.